1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus using a continuous paper transported as a recording medium and which is designed so as to prevent a color-to-color misregistration state which tends to be caused due to a variation in the transport speed of the continuous paper when respective color toner images are transferred onto the continuous paper.
2. Description of the Related Art
In a tandem type color laser printer, photoreceptor drums carrying toner images of respective colors (cyan, magenta, yellow, black, and so forth) are provided along a transport path for a recording medium. Each of the photoreceptor drums is arranged such that after a surface thereof is charged by a charger, a surface potential is decreased by means of an exposure device so that a latent image is formed thereon. The latent image is developed into a toner image by means of a development device. Then, the toner image carried by each photoreceptor drum is transferred onto the recording medium by means of a transfer device.
Among such tandem type color laser printers is one which uses a continuous paper as a recording medium.
As will be understood, it is required that toner images on respective photoreceptor drums be sequentially transferred in precise registration onto the continuous paper, thereby preventing a color-to-color misregistration state of the toner images. In order to satisfy such a requirement, in a tandem type color laser printer using a continuous paper, it has so far been proposed to apply a tension to the continuous paper so that the continuous paper is tensioned before and after the transfer, thereby preventing a color-to-color misregistration state of toner images from being caused because of the continuous paper going slack or getting wrinkled (for example, refer to JP-A No. 6-64244).
However, it has sometimes happened that a color-to-color misregistration state is caused despite application of a tension to the continuous paper The inventor has conducted an analysis of color-to-color misregstration states, and found out causes for such states as mentioned below.
When a toner image on a photoreceptor drum is transferred onto a continuous paper, since that portion of the photoreceptor drum's surface where no toner image is formed is charged, the continuous paper is electrostatically attracted to the photoreceptor drum. Furthermore, since a coverage of the toner image (a proportion of area occupied by the toner image) differs from one photoreceptor drum to another, the electrostatic force of attraction of the continuous paper also differs from one photoreceptor drum to another.
For this reason, a tension applied to the continuous paper between adjacent ones of the photoreceptor drums is varied so that tensions applied to the continuous paper between the respective photoreceptor drums become nonuniform. Consequently, the transport speed of the continuous paper is varied, and the variation in the transport speed of the continuous paper causes a color-to-color misregistration state in a transfer process.